1. Field of the Invention
This invention relates to analog-to-digital converters. More specifically, this invention relates to analog-to-digital converters employing sigma-delta modulation.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
Recent developments in the field of digital signal processing, particularly within the areas of radar, digital radio and digital television, have accentuated the demand for fast, accurate analog-to-digital (A/D) converters. For analog-to-digital converters, accuracy may gauged by measuring the signal-to-noise ratio of the output generated by the converter. The result is often expressed as resolution in terms of a particular number of bits. Typically, either a successive approximation or a dual-ramp conversion technique is used for high (i.e. 16-bit or greater) resolution A/D converters.
One difficulty with the successive approximation approach is that trimming a weighting network associated therewith is necessary to achieve a conversion accuracy in excess of 15 bits. The requirement of trimming inhibits production efficiency and increases unit costs.
High resolution is effected through the dual-ramp technique by utilizing, for example, precision high-speed integrator and sample-and-hold circuits. These circuits are generally realized only in certain specialized bipolar process technologies and then only with some difficulty.
Accordingly, A/D conversion techniques based on "oversampling" have been viewed favorably since this methodology obviates the need for trimming and for certain precision circuits. A/D converters utilizing oversampling operate at a clock rate much larger than the data rate of the sampled analog signal to be processed. The oversampling ratio of an A/D converter refers to the ratio of the clock rate of the A/D converter to the Nyquist sampling rate associated with the incident analog signal. As is well known, the value of the Nyquist rate is dependent upon the maximum frequency of interest included within the incident analog signal.
One class of oversampling A/D converters is based on a processing scheme known as sigma-delta modulation. Conventional sigma-delta modulators output a bit stream having a pulse density proportional to the amplitude of the applied input signal. In sigma-delta A/D converters the sigma-delta modulator is typically followed by a decimating digital low-pass filter. The digital filter produces a more conventional "digital word" representation of the analog input at a lower sampling rate than that of the modulator.
Unfortunately, the large oversampling ratios characterizing existing sigma-delta A/D converters limit the analog signal bandwidth which may be accurately processed by a single converter. As a consequence, conventional sigma-delta A/D converters have typically been constrained to applications involving, for example, audio signal processing. Further, the oversampling ratio is inversely proportional to the speed at which an analog signal may be converted to the digital domain. It follows that the large oversampling ratios of conventional sigma-delta A/D converters may preclude the inclusion of these converters in certain high speed processing applications. For example, a cascade of three first order sigma-delta modulators employing 1-bit quantization and oversampling ratios in excess of 64 to achieve 15 to 16 bit resolution are generally required.
Hence, a need exists in the art for a precision sigma-delta A/D converter capable of high-speed data conversion at a sampling rate allowing for a relatively low oversampling ratio.